U.S. Pat. No. 4,397,270 (=JP-A 55-137305) discloses a variable valve timing and lift system. It includes a driving shaft, a control rod with axially spaced eccentric cams, and a pivot structure. The pivot structure supports valve operating (VO) cams for pivotal motion above valve lifters of cylinder valves. Springs are mounted for the VO cams, respectively. Each of the springs biases one of the corresponding rocker cams toward its rest position where the associated cylinder valve closes. Rocker arms operate the VO cams, respectively. The eccentric cams, which are in rotary unison with the control rod, bear the rocker arms, respectively. An axis of each of the eccentric cams serves as the center of drive of the corresponding one of the rocker arms. Cams fixed to the driving shaft operate the rocker arms, respectively. An electronic control module (ECM) is provided. Sensors on the engine send information on engine speed, engine load, vehicle speed, and coolant temperature to the ECM. At a predetermined swithover point, the ECM sends a signal to an actuator for the control rod. As the actuator turns the control rod, the eccentricity of each of the eccentric cams with respect to an axis of the control shaft changes. This alters the position of pivot center of the rocker arms relative to the position of pivot center of the VO cams. This causes variation in valve timing and lift of each of the cylinder valves.
According to this known system, the driving shaft is not mounted above the cylinder valves. This arrangement has a potential problem that the considerable modification of the conventional overhead camshaft engine is required to install the driving shaft. Besides, the pivot structure and driving shaft requires a considerable space to install.
the driving arrangement in which the rocker arms press the VO cams against the springs confines an allowable angle through which the VO cams can pivot within such a relatively narrow range as to ensure that the rocker arms will not disengage from the VO cams.
According to the driving arrangement, the springs maintain contact of the VO cams with the rocker arms. This contact cannot be maintained when the driving shaft rotates at high speed due to inertia of the springs. This causes the occurrence of undesired motion of the cylinder valves.
An object of the present invention is to provide a VVA apparatus, which may be mounted to the conventional overhead camshaft engines without any considerable modification of the cylinder heads.